The U.S. space agency NASA says the next decade poses challenges as great as those engineers and scientists faced two generations ago when the space program was in its infancy. Despite recent setbacks, NASA has long-term plans to keep the United States at the forefront of space exploration.

It has been a difficult few years for NASA. U.S. manned space flights were halted for more than two years after the space shuttle Columbia disintegrated over central Texas in 2003 and the launch of another space shuttle has been postponed until at least July.

In testimony Tuesday, NASA Administrator Michael Griffin asked the Senate to support the agency's nearly $17-billion budget proposal for 2007. He said the new budget is designed to meet President Bush's goal of finishing the International Space Station.

Griffin says if NASA is to take on even bolder missions in the future, such as voyages to Mars, completing the space station is crucial, as it will afford scientists the opportunity to conduct experiments that would make such a voyage possible some day.

"We will use the station to study more carefully the effects of zero gravity on humans in space in preparation for longer voyages of exploration," said Griffin. "Most crucially to me as an engineer, we will use the station as place to learn how to live and work in space."

The NASA chief also says he believes the Space Shuttle Discovery is on track to launch this summer.

"We believe that we will be in good shape to fly this July," he said. "We believe that when we fly, we will fly well."

NASA estimates that it will take 16 shuttle flights to finish assembling the International Space Station, and one additional flight to service the Hubble telescope.

The space agency is also working on a Crew Exploration Vehicle - or CEV - which would ferry crews of up to six astronauts and their cargo into space. Griffin says he does not think NASA will have a CEV before 2011 or 2012.

The NASA chief also says he has accepted an invitation from China to visit that country and discuss possible space cooperation. China launched its first manned space mission in 2003, and says it hopes to land an unmanned probe on the moon by 2010.